Rename 3.5-dev to 4.0-dev

The upcoming version has been updated from Ruby 3.5 to Ruby 4.0.
ruby/ruby@6d81969

A Ruby built with ruby-build's 3.5-dev already shows version 4.0.0dev.

```console
$ rbenv install 3.5-dev
(snip)

$ ruby -v
ruby 4.0.0dev (2025-11-08T11:52:39Z master 4365c4fb6b) +PRISM [arm64-darwin24]
```

Therefore, it would make sense to make it installable with `rbenv install 4.0-dev`.

NOTE: Since there was no `rbenv install 2.8-dev` before it was updated to `rbenv install 3.0-dev`,
`3.5-dev` is not kept either.

Author: koic